Early ambulance cots were cloth stretched between two long poles. Adding four wheels made cots easier to move to ambulances or fire and rescue trucks. Two additional wheels not in contact with the ground were eventually added to the loading end to make it possible to wheel the cot to the vehicle and engage the extra wheels with the floor of the vehicle before taking the weight off of the ground wheels. However, this wheel assembly, although providing for added mobility on flat surfaces, is not well suited for stairs. In addition, as the cot is wheeled down the stairs, the patient is inclined and the vibration of the wheels bouncing down each stair not only causes pain, but may also result in further injury if the patient is inadvertently allowed to slide off of the cot, particularly if not properly restrained.